Lock rings are provided on tamper-proof closures so as to provide externally visible evidence of an unauthorized opening of a screw cap for the removal of the contents of a container. For this purpose, identations, which break open all at the same time when the cap is unscrewed, are uniformly distributed over the circumference of the lock ring. However, as a result of the simultaneous breaking open of several indentations distributed over the lock ring, enough clearance is formed so that the lock ring can be stretched sufficiently without actually severing the individual parts of the ring. The integrity of the lock ring is preserved, so that the ring parts which are still attached together can be squeezed together again after reclosure. Thus, any reliable indication of an unauthorized opening of the container is largely defeated.
In an attempt to overcome such problems, Offenlegungsschrift 2,607,991 discloses a tamper-proof closure created such that the complete severance of only one ring part definitely occurs when the screw cap is unscrewed. Therefore, no possibility exists of imitating an original closure by squeezing together the ring portions which have been broken open.
Since only one snap-off section is provided and the remainder of the ring remains closed, the bursting force is concentrated in only two indentations. There is no other way for the ring to yield. Since the opening of the indentations takes place successively, the break-away section is removed from the ring and is discarded as a whole. Thus, it is no longer possible to put the separate parts of the ring back together again.
However, a disadvantage results from the complicated construction of the very shallow lock ring which, in the rough container filling operation, can be unintentionally broken if it is not carefully placed on the neck of the container prior to screwing the screw cap on. Even when the ring is simply grasped by gloved and therefore comparatively insensitive hands of the operator, breakage of the ring is always possible. Furthermore, the lock ring is placed in the locked position when the screw cap is turned. The lock ring is held against rotation by separate tongues which yield in the tightening direction but hold fast in the unscrewing direction by engaging stop projections. The delicate tongues are broken off if the screw cap is tightened and the position of the ring is not precisely adjusted.
Applicants have invented a tamper-proof closure which overcomes the above-noted limitations of the prior art.